If you have everything fitted, holes predrilled, and you have dry assembled the cab with screws, you’ll be fine. Once the pl premium has set up, you can remove the screws and round over edges, if you are going that route. Or you can leave them in.
The screws do the same job as the clamps. The biscuits will help keep things lined up as well. The baffle and back, and port shelf, hold things square. Assembling the baffle and mid chamber first also help.
The braces and spine also help. (If they are in right). Personally, I ad bracing after I build the box.
You can use scrap wood to screw down blocks to your assembly table, (plywood on sawhorses) to hold the cab, and use wedges to tighten. You can also screw the cab down to hold it tight while you are working.
Roof rack straps, ratchet straps, even bailing twine twisted with a stick, can help.
It’s not rocket surgery, but you want to be as prepped as possible before the glue comes out. Pl is slippery stuff, but you have a fair amount of working time. Practice putting it together a few times.